It is known to transmit electronic voice, video, and data signals through communications networks, such as the public switched telephone network (PSTN), the internet, and private local area and wide area networks (LANs and WANs). The network communications medium may be wireline, such as coaxial cable, twisted pair, or fiber optic cable, or wireless, such as cellular or radio frequency (RF) transmission. The majority of installed networks are wireline, and the most common type wireline is unshielded, twisted pair copper wire.
In operation, the networks provide simultaneous shared access by different users, and the signals appear in both digital and analog format; often together, such as in the PSTN where they are transmitted simultaneously in different segments of the network""s frequency bandwidth. Also, the different network digital devices, such as computers, telephones, and video displays, have different signal bandwidth (bit per second) requirements. They may even have different transmission and reception signal bandwidths, as in the case of internet communications or data retrieval operations, where the download data volume far exceeds the upload commands. This is accommodated by using different digital transmission protocols, such as asymmetrical digital subscriber line (ADSL) and integrated-services digital network (ISDN), which PSTN service providers use for internet communications. In summary, a single conductor pair may simultaneously carry several full duplex signal exchanges, each at different frequency bandwidth segments, with different digital signal bandwidths, and possibly combined analog and digital format.
Networks, other than LANs, are interconnected to provide out of network communications. The interconnections are provided through the use of bridges and/or routers for the internet protocol (IP) networks, and by local and central office telephone switches for the PSTN. All of the interconnected network signal traffic flows through these switch points. Since it is necessary to monitor network signal traffic to determine performance trends or to isolate and repair failures, it is necessary for both IP network and telephone technicians to determine the presence of a particular signal format (analog or digital) on a line, as well as to monitor the quality of the signal. This quality monitoring includes auditing both the tonal quality of audio transmissions as well as the transmission fidelity of a particular digital protocol. Network quality standards, such as the BELLCORE standards for the PSTN, require that the monitoring and fault isolation occur with minimum signal disruption. Ideally, therefore, the tools used by the technician must be both efficient in locating the signal, and non-disruptive of signal traffic.
The prior art discloses various types of signal analyzers which determine the presence and transmission protocol of network signals. The manner in which these analyzers access the network signals varies. Some are invasive in that the sensor makes physical electrical contact with the conductor pairs by piercing the wire insulation with pointed probes. One such type used by telephone network technicians is referred to as a xe2x80x9cbed of nailsxe2x80x9d in that it includes a number of fixture mounted probes which engage and make physical contact with the conductor wires. This physical contact creates signal noise which may manifest itself to the network user as audible sounds in the case of audio signal transmission or which may result in noise interference sufficient to interrupt and terminate a digital signal transmission. While audible interference may be annoying, the termination of a digital transmission and resulting loss of data may have much greater consequences.
There are also prior art non-contact sensors which inductively couple the transmission signal from the conductor pair, avoiding the problems resulting from physical connection. The inductively coupled sensors, however, also have disadvantages. One disadvantage is its limited signal sensitivity. These are current sensitive devices which are appropriate for use in detecting high current signals, but have limited use in low power signal detection such as is the case with network and telephone signals. A second disadvantage is the back electromagnetic force (emf) generated by the pick-up inductor (or coil) into the signal stream. This appears as noise on the line, which has the possibility of disrupting the signal or distorting its waveshape.
There is, therefore, a need for a non-invasive signal detector capable of providing high signal resolution coupling of low power (low level) line signals without interfering with the signal""s transmission.
One object of the present invention is to provide a signal sensor capable of the detection of analog and/or digital signal transmissions over a conductor without perceptible affect to the signal quality and without disrupting the transmission integrity. A further object of the present invention is to provide a sensor capable of providing high fidelity signal capture without the need to make physical contact with the transmission carrying conductor. A still further object of the present invention is to provide a non-contacting, high fidelity sensor which is capable of performing signal capture of a variety of different digital signal protocols, as may be required for use in conjunction with various model protocol analyzers. A still further object of the present invention is to provide such a non-contacting, high fidelity signal sensor in a hand-held configuration which may be easily manipulated and used by a technician.
According to the present invention, a non-contact sensor includes a capacitive probe having at least one electrically conductive plate which, when placed in proximity to a signal carrying conductor, capacitively couples a sample of the signal transmitted on the conductor to analyzer circuitry which identifies the sampled signal format. In further accord with the present invention, the signal sensor includes signal conditioning circuitry, which is intermediate to the conductive plate and the signal analyzer circuitry, to provide a high impedance termination to the capacitively coupled sample signal and a low sample signal source impedance to the signal analyzer circuitry, thereby providing the sensor with a high degree of signal sensitivity.
In still further accord with the present invention, the signal sensor includes an electrically conductive shield which is placed proximate to, but spaced apart from, the conductive plate, the conductive shield being electrically connected to the signal output of the signal conditioning circuitry so as to maintain the shield at substantially the same voltage potential as that of the conductive plate, thereby shielding the plate from environmental electrostatic effects to further enhance the sensor signal sensitivity. In yet still further accord with the present invention, the sensor includes a capacitive probe having first and second electrically conductive plates, each adapted for placement in proximity to an associated one of a pair of signal conductors to capacitively couple a sample of the associated conductor transmitted signal, the sensor further including differential signal conditioning circuitry for providing an output signal to the analyzer circuitry which is proportional to the difference sum of the first and second plate coupled signals, thereby further increasing the sensor signal to noise ratio. In yet still further accord with the present invention, the size and geometry of the probe conductive plates are selectable, and are not limited to planar devices, but may themselves comprise wire conductors.
The sensor of the present invention may be provided in either a single plate sensor configuration and a double plate sensor configuration. The double plate configuration permits use of differential mode signal detection and conditioning, thereby rejecting common mode noise present in the single plate configuration, and providing a higher degree of signal fidelity as well as higher signal to noise ratio. However, the single plate configuration provides a suitable degree of detection accuracy and signal fidelity for both analog and digital signal formats, but at a lower signal to noise ratio. Because this embodiment does not require two sensors, the differential signal conditioning circuitry is not required, resulting in overall lower cost.
The conductive plates, whether in the single or double plate embodiment, are shielded from stray electrostatic effects by conductive shields placed in spatial proximity to each plate. The shields are maintained at substantially the same voltage potential as that of the conductive plates to minimize any capacitive effect resulting from their spatial proximity. The electrical connection of the shields to the low output impedance of the signal conditioning circuitry also routes incident noise to signal ground; away from the plates. In addition, the electrical signal path, e.g. the printed circuit board (PCB) conductive trace, from the plate to the signal conditioning circuitry may also be shielded from stray effects by a xe2x80x9ctrace shieldxe2x80x9d. The trace shield is deposited on the PCB, between the signal carrying trace and the signal conditioning circuitry, and is electrically connected to the electrostatic shield. It is, therefore, at the same voltage potential and has the same low impedance signal return as the shield, and its effect is to prevent current leakage through the PCB, from the signal trace to the signal conditioning circuitry, thereby maintaining the sensed signal strength.
The sensor""s analyzer circuitry is capable of detecting both analog and digital signals over a wide frequency range and among several digital signal protocols. These include about 300 Hz to about 3200 Hz for analog voice signals, about 25 KHz to about 1100 KHz for ADSL, about 28 KHz to about 772 KHz for T 1 signals, and about 28 KHz to about 1,024 KHz for E 1 signals (the European equivalent standard for T1), or an overall sensor bandwidth of 1100 kHz.
These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying Drawing.